One example of a manufacturing process that requires controlled evacuation and repressurization of a work station is the process of controlled doping of semi-conductor wafers with ions in an ion implantation chamber. Ions from a source are accelerated along an ion travel path to impinge upon the wafers and introduce controlled doses of impurities into the silicon wafer. The ion travel path must be evacuated to assure the ions are well collimated. To accomplish this process in the prior art, wafers have been introduced to an ion implantation chamber, either through a load-lock or by introducing the wafers directly into the implantation chamber. If a load-lock arrangement is used, the load-lock chamber is successively evacuated and pressurized as wafers are inserted into the load-lock on their travel path to the ion implantation chamber. If no load-lock is used, the wafers are inserted directly into the ion implantation chamber which itself must be pressurized, evacuated, and then repressurized as the workpiece are inserted into the chamber, treated and then removed.
Other examples of processes involving pressurization and depressurization of a chamber are known in the prior art. In a sputter coating procedure, for example, workpiece are inserted into a treatment chamber and then a coating is applied to the surface of the work piece by sputtering the coating material away from a target. This procedure can be used, for example, in coating magnetic material into a recording medium. Again, prior to conducting the coating process, the work piece must be inserted into the chamber and then the coating process conducted at a reduced pressure.
It is often a requirement in these processes that the level of contaminants within the processing chamber is kept at a minimum. If the contaminant level in a doping chamber, for example, exceeds a specified value, the semi-conductor yield of the process will be reduced.
Although steps are taken to reduce the level of particulate contamination within a processing chamber, these steps cannot totally avoid such contaminants. Particulates are inevitably introduced, for example, as the workpieces are inserted into an ion implantation chamber. These particulates tend to settle on the interior walls of the chamber and remain in place until air flow that occurs during chamber evacuation and repressurization dislodge the particles causing them to move within the chamber. If the particulate contaminants remain attached to the chamber walls, the work piece can be inserted into the chamber, treated, and removed without undue contamination. It is when the particles are dislodged and come to rest on the workpiece either before or after the ion implantation process that the particles' presence reduces production yield.
Prior art U.S. Pat. No. 4,739,787 to Stoltenberg which issued Apr. 26, 1988 recognizes the possibility for contaminant presence effecting the yields during semi-conductor wafer fabrication. This patent recognizes the possibility of dislodging contaminants from chamber walls as air enters and exits the process chamber. As a proposed solution to the dislodging problem, the '787 patent recommends the use of "soft-start valves" which open in accordance with a controlled profile so that "turn on" turbulence is reduced. Specifically, the '787 patent calls for a pressurization and depressurization of a chamber in accordance with a time profile.
While the '787 patent recognizes the importance of reducing detachment of particles from the chamber wall, this patent makes no mention of controlling flow velocity. Instead, the patent focuses on avoiding turbulence and accomplishes this by pressurizing and depressurizing the chamber in a timed sequence which avoids gas flow turbulence. The present invention concerns a type of pressure control for the pressurizing and depressurizing of a chamber to achieve appropriate pressures in an efficient manner without dislodging undue amounts of contaminants from the chamber wall.